fix: inaccurate Server-Timing durations

The transaction duration was notably off, doing:

```
curl localhost:3000/rpc/sleep?seconds=5 -i
```

Shows `46.1` for the `transaction;dur`, with this fix we obtain
`5007.3`.

Fixes https://github.com/PostgREST/postgrest/issues/4522

This also fixes inaccurate "schema cache queried" logs,
see https://github.com/PostgREST/postgrest/issues/4551.

src/PostgREST/TimeIt.hs

@@ -0,0 +1,20 @@
+module PostgREST.TimeIt
+  ( timeItT
+  ) where
+
+import GHC.Clock
+import Protolude
+
+{-
+ - The signature is the same as https://hackage.haskell.org/package/timeit-2.0/docs/src/System-TimeIt.html#timeIt,
+ - we vendor this functionality because it gave errors as shown on https://github.com/PostgREST/postgrest/issues/4522 plus
+ - the function is small enough. This vendored function is different in that the result is in milliseconds.
+ -}
+timeItT :: MonadIO m => m a -> m (Double, a)
+timeItT p = do
+  s <- liftIO getMonotonicTime
+  x <- p
+  e <- liftIO getMonotonicTime
+  let time = (e - s) * 1000
+  return (time, x)
+